The present invention relates to the field of seismic exploration. More particularly, the invention relates to a method and apparatus for the control and correction of the time base used in a distributed nodal seismic acquisition system.
Seismic exploration generally utilizes a seismic energy source to generate an acoustic signal that propagates into the earth and is partially reflected by subsurface seismic reflectors (i.e., interfaces between subsurface lithologic or fluid layers characterized by different elastic properties). The reflected signals (known as “seismic reflections”) are detected and recorded by seismic receivers located at or near the surface of the earth, thereby generating a seismic survey of the subsurface. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.
Typically, the seismic receivers are laid out in an array, wherein the array consists of a line of stations each comprised of strings of receivers laid out in order to record data from the seismic cross-section below the line of receivers. For data over a larger area and for three-dimensional representations of a formation, multiple single-line arrays may be set out side-by-side, such that a grid of receivers is formed. Often, the stations and their receivers are spread apart or located in remote areas. In land seismic surveys for example, hundreds to thousands of receivers, called geophones, may be deployed in a spatially diverse manner, such as a typical grid configuration where each line extends for 5000 meters with receivers spaced every 25 meters and the successive lines are spaced 500 meters apart. Depending upon many geophysical factors, as well as operational down time due to equipment or weather conditions, the spread units may be deployed for time intervals in excess of two weeks.
Acoustic waves utilized in seismic exploration are typically generated by a centralized energy source control system that initiates an energy event via a dynamite explosion, air gun shot, vibrator sweep or the like. The acquisition system, i.e., the seismic receivers and their control mechanism, is synchronized to the energy event such that the first data sample of the acquisition period corresponds in time to the peak of the energy event, such as the start of a sweep for vibratory operations. Acquisition periods typically last between 6 to 16 seconds following the first sample, with each seismic sensor being sampled at an interval between 0.5 to 4 milliseconds.
Of fundamental importance to any seismic system is the time base method by which the synchronization of the energy event and the sampling of the acoustic wave field is accomplished. FIG. 1 represents the principal elements involved in a typical prior art a seismic acquisition system 10 which is connected via a hardwire 12 to a plurality of individual seismic data acquisition sensors 14. The elements are utilized to control the time base and distribute the time base to each individual seismic data acquisition sensors 14, thereby permitting the overall system 10 to be time synchronized. As shown, the prior art uses a single, centralized time base which insures that all individual seismic data acquisition sensors 14 are sequenced during the acquisition cycle by the same time reference. The synchronization time reference is maintained at a centralized base unit 16, such as an operation management vehicle. This time base is typically disciplined by a continuously operated wireless receiver 18, such as a global positioning system (“GPS”) receiver, which is disposed to communicate with an external time reference 20, which in the case of a GPS receiver are GPS satellites. The GPS receiver 18 directly disciplines a high stability voltage control oscillator (“VCO”) 22 that is used to drive the system clock 24 to which all elements are typically phased-locked. The acquisition system controller 26 utilizes a Phase-Locked-Loop (PLL) to synchronize its outbound command frames to the system clock 24. The outbound command frames are in turn locked onto by the PLLs in the plurality of seismic data acquisition sensors 14 cabled to the acquisition system controller 26. Embedded in the command frames is the sample clock signal used to synchronize the analog-to-digital (A/D) converters 28 in the sensors 14 to the GPS signal, which is typically 1 Pulse-Per-Second (1 PPS) signal or any time interval that is an integer multiple of sample intervals following that time epoch. In any event, the energy source controller 30 is synchronized to the system clock 24 via discrete hardware interfaces that are either directly connected to the centralized GPS disciplined clock 24 or will utilize a PLL locked on to the central timing reference provided by the system clock 24. It is important to note that most prior art source control systems do not utilize GPS disciplined time bases to perform timing functions, but rather, use GPS time tags to time stamp certain significant events recorded by the system, such as reception of the FIRE event or the TIMEBREAK event (which represents the time of the peak source energy event) or the start of a vibratory sweep. The prior art acquisition system controller steers the time at which the FIRE event occurs to insure that the TIMEBREAK event occurs at a time synchronous with an A/D conversion of the spread seismic sensors, as required for accurate wave field sampling.
In contrast to the hardwired, centralized time base system of FIG. 1, more recent prior art seismic acquisition systems have attempted to eliminate or minimize cabling between the centralized base unit and individual seismic data acquisition sensors. In such cases, the seismic sensors are integrated with other hardware in individual seismic data acquisition units or nodes, such that some of the control and operational functions previously carried out by the base unit are now performed at the individual seismic data acquisition units, such as timing functions. In certain of these “nodal” prior art systems, each seismic data acquisition unit continues to communicate wirelessly with the centralized base, whereas in other “autonomous” nodal prior art systems, each seismic data acquisition unit operates independently of the centralized base.
The principal elements involved in a typical prior art “nodal” seismic acquisition system that utilizes autonomous seismic data acquisition units are similar to the block diagram shown in FIG. 1, except that physical layer connection (either wired or wireless) between a centralized unit and the field spread of seismic units is eliminated, such that the individual seismic acquisition units operate at least semi-autonomously from the central unit. In the case of elimination of a wired physical layer connection, many of the drawbacks arising from cables are eliminated, such as weight, cost and high failure rates. Likewise, in the case of elimination of a wireless physical layer connection, many of the drawbacks arising from a wireless connection are eliminated, such as bandwidth limits, susceptibility to interference, and the need for radio channel licenses.
These autonomous seismic acquisition units are characterized by one or more seismic sensors that are deployed in a spatially distributed array about the node. Each individual sensor is in communication with the node via a cable. Commonly, multiple sensors are wired to a single cable to create an array.
One significant improvement in autonomous seismic data acquisition is the development of fully integrated, self-contained autonomous seismic acquisition units, such as those described in U.S. patent application Ser. Nos. 10/448,547 and 10/766,253. In these applications, there is described a continuous recording, self-contained, autonomous wireless seismic acquisition unit. The self-contained unit comprises a fully enclosed case having a wall defining at least one internal compartment within the case; at least one geophone internally fixed within said internal compartment; a clock disposed within said internal compartment; a power source disposed within said internal compartment; and a seismic data recorder disposed within said internal compartment, wherein each of said electrical elements includes an electrical connection and all electrical connections between any electrical elements are contained within said case. Thus, unlike the prior art, the seismic sensors or geophones, are also contained within the case itself, rendering the entire system self-contained and eliminating external wiring or cabling of any type. The case is shaped to enhance deployment and coupling with the ground by maximizing the surface area of the case in contract with the ground. Preferably, the case comprises a first plate having a first periphery and a second plate having a second periphery, wherein the plates are joined along their peripheries by the wall defining the internal compartment. As such, the case may be disk shaped or tubular in shape. Such a unit is desirable not only for the shape of the case, but also because being fully self-contained, external cabling, such as between an electronics package and a seismic sensor/geophone, are eliminated.
In any event, when the physical layer connection with a centralized unit is eliminated, the autonomous seismic units must be implemented with a distributed time base, meaning that a control clock system is disposed on each individual seismic unit. Moreover, without a cable connection for synchronization or data telemetry, autonomous nodal seismic systems must rely on the use of battery based power sources for the individual seismic unit electronics. Wireless seismic acquisition units such as these operate independently of the energy source control system and the timing clock associated therewith. Rather, they rely on the concept of continuous acquisition of a timing signal, and in the case of the referenced patent application above, the continuous acquisition of data as well. Knowing that the source event is synchronized to the sample interval of the seismic data, the data can be associated with the correct source event in a non-real time process following the retrieval of the node.
With the elimination of the physical layer connection for distributed wireless seismic acquisition units, the manner in which each seismic unit's sample clock is derived and the synchronization of that sample clock with the energy source events must address the loss of the command frame synchronization of the prior art system in FIG. 1.
In the prior art, autonomous seismic acquisition units commonly synchronize and discipline their local time bases using the same method and apparatus implemented by the centralized time base architecture systems. Specifically, synchronization is accomplished by implementing a wireless interface to a continuous, common time reference, such as a GPS system of satellites. In such case, the GPS satellite time base is utilized as the system clock via a GPS receiver installed on board each individual seismic acquisition unit as opposed to a GPS receiver installed on board the centralized unit. However, such a time base system for autonomous units is undesirable for a number of reasons.
First, systems with continuously operating functions, such as a clock, utilize significant amounts of power. While a centralized unit may have access to a continuous power source, autonomous seismic acquisition units do not, but must rely on power source with limited capacity, namely a battery. Specifically, the use of a continuously operated wireless receiver to discipline a VCO is very power inefficient. For example a continuously operated GPS receiver could consume between 20 to 50 percent of the total battery power of a seismic unit. To address this, prior art acquisition systems most commonly utilize the “stand alone” node described above, wherein a plurality of seismic sensors are deployed in a spatially distributed array about the node, with each sensor in communication with the node via a cable. While such systems distribute the power load of a continuously disciplined clock across multiple seismic sensors, such a system reintroduces the use of unreliable cables to connect the spatially distributed seismic sensors. As the number of seismic sensors connected to an acquisition unit approaches one, however, the percentage of the total power budget of the unit utilized to maintain wireless synchronization becomes much more significant and power becomes a limiting factor governing the deployment length of the seismic acquisition unit.
Second, wireless access to the external time reference 20, will be significantly more difficult for nodal acquisition seismic units as compared to a receiver at a centralized base unit, such as a recording truck. The wireless receiver and antenna of a nodal seismic acquisition unit is located within the unit itself (or in close proximity thereto) and such units are generally deployed close to the ground (or in some cases may actually be below the ground surface). Moreover, physical placement of the unit is dictated by the geometry of the spread itself, and hence, physical placement cannot be altered to achieve better wireless access. Further, heavy foliage, rugged terrain and urban obstructions can all contribute to limiting the ability of the nodal wireless receiver to maintain a continuous timing solution. The result is that a continuous external time reference signal from a GPS satellite or other source is likely to be disrupted and intermittent over the course of a shoot. In contrast, a base unit such as a recording truck can generally be positioned in a location where wireless access to the time reference is unobstructed and not an issue.
With limited wireless access to the external time reference 20, the nodal time bases must rely on the stability or “holdover” capabilities of the VCO in the control loop to maintain a stable frequency output during periods when the control loop does not have a continuous reference to discipline the VCO. One prior art solution utilizes high stability ovenized or atomic based oscillators acting as the “holdover” time base. However, the cost and power requirements for such oscillators makes their use impractical. A more typical solution is to use a high stability, temperature compensated quartz oscillator as the “holdover” oscillator. This class of VCO can maintain a fixed frequency within ±5E-7 over the industrial operating range of a node.
A third drawback to implementation of an autonomous seismic acquisition unit utilizing a continuous GPS receiver as the system clock arises from the manner in which the wireless receiver corrects the frequency of the VCO following long periods of poor wireless availability. Current prior art methods cause distortion in the A/D process of the delta-sigma converters used in such acquisition units. The control loops implemented in these prior art GPS disciplined time bases are designed to steer the 1 PPS output of the disciplined clock to align with the GPS 1 PPS signal. This is accomplished by varying the frequency of the VCO to compensate for the time difference between the two 1 PPS references. The attack rates at which this frequency correction is performed is designed to minimize the time over which the correction is made so that the disciplined clock is rapidly brought back into synchronization with GPS time reference. While these GPS disciplined time bases typically allow some limited control of the attack rate of the control loops, thus providing some reduction in the distortion caused by the change in the VCO operating frequency, this reduction in the attack rate greatly increases the time interval over which the correction is made and over which the GPS receiver must remain in a high power consumption state.
There exists the need to establish a method by which autonomous nodal seismic acquisition units, distributed over wide spatial areas, can be synchronized to each other and to a seismic energy controller while minimizing power consumption of the units. Such a method must address the lack of either a wired or wireless physical layer connection between nodes or a control unit and must do so in a low power manner. The apparatus used to implement the time base interface to an external time reference, such as GPS, account for the intermittent and unreliable nature of the time base due to operational and environmental variables within which the unit must function. As such, it would be desirable to have a control loop design to implement the time base so as to stabilize oscillator performance when access to an external time reference is not possible. Control loop algorithms should adapt to oscillator performance characteristic and predictive methods should be used to avoid the need to access the external time reference during periods when there is a low probability of successfully connecting to the external time reference.